1. Field of the Invention
This invention relates generally to oilfield well testing and more particularly to production testing of wells wherein fluid from a production zone is injected into a another subsurface zone.
2. Description of the Related Art
After drilling of a well to a known depth, a production zone or zones are identified by a variety of known techniques. xe2x80x9cProduction testxe2x80x9d or xe2x80x9cproduction testingxe2x80x9d is carried out to obtain data to determine a variety of characteristics of the oil and gas reservoirs, including the flow characteristics of the reservoir fluid, such as permeability.
A variety of production testing methods are known. Production tests are performed prior to completing a well (in open holes) as well as in cased or completed wells. Usually, a production test has two phases, each with a duration of several hours to a few days. In the beginning, the fluid adjacent the production zone flows into the well, but gradually the fluid from greater distances must flow into the well. The pressure in the well decreases because the fluid must flow over a longer distance through the formation, subjecting it to increasing pressure loss. When a constant flow rate from a particular zone is maintained, then the pressure in the well depends only on the character of the formation. During the first phase of a production test, pressure and temperature measurements over time are recorded, during constant flow rate. In the second phase of the production test, the fluid flow from the production zone being tested is stopped. The pressure within the well then gradually rises to the formation pressure as the formation around the well is filled with the fluid from the remote areas. The pressure build up over time and temperature overtime are recorded. The pressure overtime, temperature over time and the flow rate measurements are most commonly used to analyze the reservoir characteristics.
During the first phase of the production testing, the reservoir fluid is conducted to the surface via a tubing. Packers in the annulus between the tubing and the well are placed to seal the annulus so the formation fluid will flow through the tubing and not through the annulus. A flow control valve at the upper end of the tubing at the surface is used to control the flow of the fluid from the formation. Downhole pumps are sometimes installed to maintain the desired fluid flow rate. The above-described and other known production testing methods usually require flowing substantial amounts of formation fluid to the surface during the first phase of the production test. Such methods suffer from a number of disadvantages.
In open hole wells, there usually are no or very inadequate facilities at the surface to process the formation fluid brought to the surface. The reservoir fluid poses safety risks as it is flammable and hazardous to the environment. Therefore, substantial safety measures are taken in connection with such production tests. To reduce the environmental risks, the reservoir fluid is usually burned off at the well site. Combustion of hydrocarbons, however, produces unwanted gases which pollute the environment. Hydrocarbons also are often discharged into the environment. These problems are exasperated for offshore wells. In certain regions, such as the Norwegian Continental shelf, regulations restrict or prohibit burning of polluting matters. The operators in such regions collect the produced reservoir fluid and transport it to suitable offsite processing plants. Accordingly, it is increasingly becoming important to devise production testing methods which are safe, environmentally friendly and less weather dependent.
Before conducting production testing, casing is often cemented in the well to insulate various permeable layers, and to comply with safety requirements. Usually, special production tubing is used down to the layer/bed (zone) to be tested. These preparations are time-consuming and expensive. Safety considerations make it sometimes necessary to strengthen an already set casing, perhaps over the entire or a substantial part of the length of the well; particularly in high pressure wells where it might be required to install extra casings in the upper parts of the well.
It can be difficult to secure a good cementing. Channels, cracks or voids my exist in the cemented zones. In many cases, it is difficult to define or measure the quality of the cementing operation or the presence of cement. Unsatisfactory cementing can cause so-called cross flows to or from other permeable formations outside the casing. Cross flows may, to a high degree, influence the measurements carried out. Time-consuming and very expensive cementing repairs might be required in order to eliminate such sources of errors.
Systems currently used can be adequate for take care of drilling wells in deep waters, but do not provide safe and secure production testing. In deep water operations, it is difficult to remain secure when the drilling vessel drifts out of position, or whenever the riser is subjected to large, uncontrollable and not measurable vibrations or leeway. Such a situation requires a rapid disconnection of the riser or production tubing subsequent to closing the production valve at the seabed.
Further, in ordinary production it is usual to use various forms of well stimulation. Such stimulation may include injection of chemicals into the formation in order to increase the flow rate. A simple well stimulation includes subjecting the formation to pressure pulses so that it cracks and, thus, becomes more permeable. Such methods are referred to as xe2x80x9cfracturingxe2x80x9d of the formation. A side-effect of fracturing can be a large increase in the amount of sand accompanying the reservoir fluid. In connection with production testing, it may in some instances be of interest to be able to effect a well stimulation in order to observe the effect thereof. Again, the case is such that an ordinary production equipment is adapted to avoid, withstand, resist and separate out sand, while corresponding measures are of less importance when carrying out a production test.
In some cases, it is useful to be able to carry out a reversed production test, i.e., pumping produced fluid back into the production formation. However, this presupposes that produced fluid can be kept at approximate reservoir pressure and temperature. This will require extra equipment, and it will be necessary to use additional safety measures. Further, it would require transfer of the production tubing. Probably, the production tubing would have to be pulled up and set once more, in order to give access to another formation. This is time-consuming as well as expensive. Therefore, it is not of actual interest to use such reversed production tests in connection with prior art techniques. During a reversed production test, a pressure increase is observed in the well while a reversed constant fluid flow is maintained. When the reversed fluid flow is interrupted, a gradual pressure reduction will be observed in the well. Reversed production test may contribute to revealing a possible connection in the rock ground between formations connected by the channel, and may in some cases also contribute to defining the distance from the well to such a possible connection between the formations.
The present invention provides systems and methods for performing production testing in open holes and in cased holes that avoid transporting formation fluid to the surface.
A main feature of the invention is that formation fluid is conducted from a first, expected permeable formation to a second permeable formation as opposed to prior art technique where fluid is conducted between a formation and the surface. According to the invention, prior to a production test, at least one channel connection is established between two formations, of which one (a first) formation is the one to be production tested. Further, sealing devices are disposed to limit the fluid flow between the formations through the channel connection(s). When fluid flow takes place from the first to the second formation the sealing devices, e.g. annulus packers, prevent fluid from flowing between the formations, outside the channel(s).
Within the channel, flow controlling devices are disposed, which may include flow control valves and a pump, operable from the surface in order to control the fluid flow in the channel and, thus, between the formations. Further, within the channel, a flow rate sensor is disposed. This sensor may be readable from a surface location.
Additionally, sensors adapted to determine pressure, temperature, detect sand, water and the like from the surface may be disposed. Of course, several sensors of each type may be disposed in order to monitor the desired parameters at several places within the channel. As discussed, sensors for pressure and temperature are disposed within the well. Likewise, equipment for timekeeping and recording of the measured valves are positioned in the well.
During a production test, by using the flow rate sensor, the adjustable valve and, possibly, by use of said pump, a constant fluid flow is established and maintained in the channel, for fluid flowing from one formation to the other formation. Pressure and other well parameters are read and recorded as stated above. Thereafter, the fluid flow is ceased, and the pressure build up within the well is monitored and recorded as stated. This production test may be extended to a reversed flow through the utilization of a reversible pump so that fluid can be pumped in the opposite direction between the two formations.
Storing produced reservoir fluid in a formation results in the advantage that the fluid may have approximately reservoir conditions when it is conducted back into the reservoir. Further, according to the invention, well stimulating measures in the formation being production tested may be used. Fracturing may be achieved by methods known in the art. To this end, the well is supplied with pressurized liquid, e.g., through a drill string coupled to the channel. Thereafter, a production test is carried out as described above. Additionally, a reversed production test may be conducted to obtain the production testing data from two separated layers without having to remove the test string.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.